Fe-Ni Invar Alloys Research Articles

Critical behavior of disordered fccFe70Pt30alloy under high pressure

Employing the disordered local moment formalism in combination with a first-principles tight-binding linear muffin-tin orbital method we find that in disordered fcc ${\mathrm{Fe}}_{70}{\mathrm{Pt}}_{30}$ alloy the effective magnetic interaction changes from ferromagnetic to antiferromagnetic as the lattice constant is reduced from its equilibrium value at ambient pressure. This result explains recent experiments on this alloy that have claimed an appearance of the new magnetic spin-glass phase under high pressure. The value of the volume where the change in the magnetic interaction occurs is just slightly higher than the critical volume at which ${\mathrm{Fe}}_{70}{\mathrm{Pt}}_{30}$ becomes nonmagnetic, suggesting that under applied pressure the system exhibits two types of magnetic critical points. A comparison of our results with earlier theoretical studies of the fcc Fe-Ni Invar alloys reveals a similarity between the high-pressure magnetic state of ${\mathrm{Fe}}_{70}{\mathrm{Pt}}_{30}$ and the magnetic ground state of the Fe-Ni alloys at ambient pressure.

Zero thermal expansion in YbGaGe due to an electronic valence transition.

Most materials expand upon heating. Although rare, some materials expand on cooling, and are said to exhibit negative thermal expansion (NTE); but the property is exhibited in only one crystallographic direction. Such materials include silicon and germanium at very low temperature (<100 K) and, at room temperature, glasses in the titania-silica family, Kevlar, carbon fibres, anisotropic Invar Fe-Ni alloys, ZrW2O3 (ref. 4) and certain molecular networks. NTE materials can be combined with materials demonstrating a positive thermal expansion coefficient to fabricate composites exhibiting an overall zero thermal expansion (ZTE). ZTE materials are useful because they do not undergo thermal shock on rapid heating or cooling. The need for such composites could be avoided if ZTE materials were available in a pure form. Here we show that an electrically conductive intermetallic compound, YbGaGe, can exhibit nearly ZTE--that is, negligible volume change between 100 and 400 K. We suggest that this response is due to a temperature-induced valence transition in the Yb atoms. ZTE materials are desirable to prevent or reduce resulting strain or internal stresses in systems subject to large temperature fluctuations, such as in space applications and thermomechanical actuators.

Applying the two-state invar model to delta-phase plutonium

This article describes the authors’ use of the Weiss two-state model for Fe-Ni invar alloys to understand the anomalous thermal expansion of Pu-Ga alloys. Studies on thermal expansion of Pu-Ga are reviewed briefly, and the two-state invar model is described. The authors fit the available neutron-diffraction data for Pu-Ga alloys to the invar model and discuss the consequences.

Pressure-induced change of the magnetic state in ordered Fe–Pt Invar alloy

The magnetic properties of an ordered Fe 72.8Pt 27.2 Invar alloy have been investigated under high pressures up to 7.5 GPa by AC susceptibility measurements in the temperature range from 4.2 to 430 K. The Curie temperature decreased with increasing pressure and then the ferromagnetic state vanished above 6.5 GPa. A new high-pressure magnetic phase appeared at 3.5 GPa in the low temperature range below 40 K, which is quiet different from the high-pressure phase observed in disordered Fe–Pt and Fe–Ni Invar alloys. The transition temperature to this new phase increased with pressure and the new phase became more stable. This tendency resembles to the feature expected by a recent theoretical calculation.

Diffusion bonding of Si3N4-TiN composite with nickel-based interlayers

The diffusion bonding of a Si3N4-TiN composite with Ni, INVAR (Fe-Ni alloy), and IN600 (Ni-Cr-Fe alloy) interlayers has been investigated between 1100 °C and 1350 °C, under argon or nitrogen atmosphere. For the chosen bonding conditions, the Si3N4 phase of the composite reacts with the interlayer phase, leading to the release of silicon and nitrogen, whereas the TiN phase remains stable. The bonding mechanisms with nickel and INVAR (Ni-Fe alloy) interlayers are rather similar. Released silicon diffuses into the reaction layer and into the interlayer, forming a solid solution, whereas the released nitrogen remains gaseous. The bonding rate depends then on the elimination rate of nitrogen from the reaction interface. The thermal stability of these joints is very high up to 1100 °C. However, the interfacial porosity and the internal stresses created by the high nitrogen pressure are pernicious for the mechanical strength. The bonding mechanism with IN600 (Ni-Fe-Cr alloy) interlayer is rather different. The released nitrogen can form nitrides with interlayer elements (Cr, Al). Released silicon diffuses into the reaction layer and forms silicides. The joint porosity is less significant for the IN600 interlayer, which suggests a good mechanical strength. However, the formation of silicide is pernicious, because of the brittleness of these phases.

Pressure induced magnetic phase transition in Fe–Ni Invar alloy

The magnetic properties of an FCC Fe 68.1Ni 31.9 Invar alloy have been investigated under high pressure up to 7.7 GPa by AC susceptibility measurements in the temperature range from 4.2 to 400 K. Up to 5.5 GPa, the Curie temperature ( T C) decreased linearly with increasing pressure. Above 3.5 GPa, a reentrant spin glass-like phase was observed clearly in the low-temperature region below 50 K. Above 5.5 GPa, ferromagnetic (FM) phase began to collapse with increasing pressure, and then at 7.7 GPa, FM phase disappeared, while the spin glass-like phase was still seen. Our results dearly show the pressure induced magnetic phase transition from the FM phase to the spin glass-like high-pressure magnetic phase.

Enhanced strength and fatigue life of ultra-fine grain Fe–36Ni Invar alloy

The mechanical behavior and fatigue life of ultra-fine grain (UFG) Fe–36%Ni Invar alloy fabricated by severe plastic deformation (SPD) is investigated in both high-cyclic and low-cyclic regimes. A significant enhancement of mechanical properties has been achieved via equal-channel angular pressing (ECAP); the yield stress and the fatigue limit are increased by a factor of three and two, respectively, when compared with those of ordinary Invar alloy with conventional grain size. The fatigue characteristics are determined and tabulated in terms of cyclic stress–strain curves (CSSC), endurance limit, Coffin–Manson and Basquin parameters. It has been shown that the notable improvement of both low-cyclic and high cyclic properties is possible after SPD depending on the processing route.

Filling boron nitride nanotubes with metals

The authors’ endeavors over the last few years with respect to boron nitride (BN) nanotube metal filling are reviewed. Mo clusters of 1–2 nm in size and FeNi Invar alloy (Fe ∼60 at. %; Ni ∼40 at. %) or Co nanorods of 20–70 nm in diameter were embedded into BN nanotube channels via a newly developed two-stage process, in which multi-walled C nanotubes served as templates for the BN multi-walled nanotube synthesis. During cluster filling, low-surface-tension and melting-point Mo oxide first filled a C nanotube through the open tube ends, followed by fragmentation of this filling into discrete clusters via O2 outflow and C→BN conversion within tubular shells at high temperature. During nanorod filling, C nanotubes containing FeNi or Co nanoparticles at the tube tips were first synthesized by plasma-assisted chemical vapor deposition on FeNi Invar alloy or Co substrates, respectively, and, then, the nanomaterial was heated to the melting points of the corresponding metals in a flow of B2O3 and N2 gases. During this second stage, simultaneous filling of nanotubes with a FeNi or Co melt through capillarity and chemical modification of C tubular shells to form BN nanotubes occurred. The synthesized nanocomposites were analyzed by scanning and high-resolution transmission electron microscopy, electron diffraction, electron-energy-loss spectroscopy and energy-dispersive X-ray spectroscopy. The nanostructures are presumed to function as ‘nanocables’ having conducting metallic cores (FeNi, Co, Mo) and insulating nanotubular shields (BN) with the additional benefit of excellent environmental stability.

Modification of Fe–Ni Invar alloys by high-energy ion beams

Measurements of temperature dependence of AC-susceptibility were made in Fe–30.2at.%Ni Invar alloy before and after 100 MeV Xe ion irradiation up to the dose of 10 14 ions/cm 2. Measurements were performed at various angles θ between the direction of the ion beam and the external AC-magnetic field. It was found that in partially irradiated area, locally ferromagnetic parts exist at temperatures above the Curie temperature of the body where high-energy ions did not penetrate. The easy axis of the locally ferromagnetic parts was determined to be parallel to the beam direction. Those locally ferromagnetic parts can be considered to be in thin needle-like shape. This type of modification has a possibility of applications for perpendicular high-density memory and giant magneto-resistance materials.

Magnetic Properties in Fe-Ni Invar Alloys Irradiated by High-Energy Ions

ABSTRACTAnomalously large shift of the Curie temperature has recently been observed in Fe-Ni Invar alloys irradiated with high-energy heavy ions. This large effect can be attributed to the large positive magneto-volume effect essentially originated in the itinerant electron ferromagnetism in Fe-Ni Invar alloys. To investigate the mechanism of the large modification of the ferromagnetism and the structure of the modified portion, measurements of the beam energy dependence of AC-susceptibility-temperature curves have been made. It was found that the amount of the shift of the Curie temperature did not change by increasing the beam energy. On the contrary, the intensity of the susceptibility of the modified portion increased with increasing the ion beam energy. The high-density electronic excitation is considered to be responsible for the large modification of the ferromagnetism in Fe-Ni Invar alloys.

Anomalous shift of Curie temperature in iron–nickel Invar alloys by high-energy heavy ion irradiation

Fe 0.68Ni 0.32 Invar alloys are irradiated with 3.54 GeV Xe ions, and with 2.71 GeV U ions at room temperature. Measurements of the magnetic moment of specimens under AC magnetic field before and after irradiation show that the Curie temperature, T c, for the irradiated region increases with increasing the ion fluence. This phenomenon appears even at low ion fluence, implying that it is attributed to ion-induced high-density electronic excitation. It is well known that T c of such Fe–Ni Invar alloys strongly depends on the Ni concentration and the external pressure. With increasing the Ni concentration from ∼30%, T c increases, and with decreasing the lattice parameter by the external pressure, T c decreases. Therefore, the increase in T c by high-energy ion irradiation can be explained as originating from the lattice expansion and/or the composition change, which are induced by the high-density electronic excitation.

The Temperature Dependence of Work-Hardening Rate in Invar Fe-Ni Alloys

The stage I work-hardening behavior of Invar Fe-Ni alloys was studied in tension in the temperature range from room temperature to 700 K. Work-hardening rate (WHR) is a function of temperature and has a peak at about 560 K. The anomalous strength is attributed to thermally activated cross-slip.

Insulating `nanocables': Invar Fe–Ni alloy nanorods inside BN nanotubes

Here we present the results on synthesis, structural and chemical analysis of insulating boron nitride (BN) nanotubes (NTs) which have been filled with conducting Invar Fe–Ni alloy (∼60 at.% Fe; ∼40 at.% Ni) nanorods. The result was accomplished by a two-step process: (i) carbon (C) NTs containing Invar alloy nanoparticles placed at the tube tips were synthesized by plasma-assisted chemical vapor deposition (CVD) on an Invar Fe–Ni alloy substrate; and (ii) the material was heated to the melting point of the alloy (1723 K) in a flow of B 2O 3 and N 2 gases and held for 30 min. During this second stage, simultaneous filling of NTs with the Fe–Ni melt through capillarity and chemical modification of C tubular shells to form BN tubules occurred.

Two magnetic states of iron atoms in Invar Fe–Ni alloys and positron annihilation

The temperature dependence of angular correlation annihilation radiation (ACAR) in Invar Fe–Ni alloys is investigated. It is found that the ACAR distribution in the Curie temperature region T C depends on temperature. This effect is created only by those positrons that are trapped by vacancies. The effect is enhanced if the positrons trapped by vacancy-hydrogen complexes. The ACAR distribution is changed due to enhanced interaction of these positrons with 3d electrons. A simple interpretation of this phenomenon can be given on the basis of the model of two magnetic states of Fe atoms in Invar alloys. According to this model the enhancement of the electron–positron correlation interaction in the T C region occurs as a result of the convergence of the energy levels ε HS and ε LS corresponding to the high-spin (HS) and low-spin (LS) states of Fe atoms.

Magnetization Process in Mechanically Alloyed Fe-Ni Invar

By meclianically alloying and successive heat treatment, Fe-Ni Invar alloys around 35 at% Ni have been made with wide variety of concentration fluctuation. Observed magnetization curve changed significantly by annealing. The saturation magnetization decreased first, took a minimum at 500°C annealing and then increased further increasing annealing temperature. By assuming Gaussian-type fluctuation, observed magnetization curves could be explained.

Analysis of the decomposition of Cu–Ni and Fe–Ni alloys at lower temperatures by atom probe

The decomposition of alloys can lead to concentration fluctuations with small amplitude and spatial extension. A technique of data evaluation, the repeated smoothing procedure (RSP), is described by which atom probe data of such alloys can be analysed. The procedure is applied to a Fe–Ni Invar alloy, annealed at 625°C, and to two Cu–Ni alloys, annealed at 300°C and at 350°C. Decomposition was observed and quantified in both alloy systems.

Magnetic moments of iron atoms in Invar Fe–Ni alloys

It is shown that the model of two magnetic states of Fe atoms in Invar Fe–Ni alloys allows a comprehensive interpretation of effect of deviation of average magnetic moment per atom 〈 m( c Fe)〉 from the Slater–Pauling curve. In the model under consideration this effect arises due to transition of some Fe atoms from the high-spin (HS)-state into the non-magnetic (NM)-state as a result of electron transfers between Fe–Fe pairs. It is also necessary to take into account the dependence of the lattice constant a of these alloys on their composition. Rapid decrease in 〈 m( c Fe)〉 of Invar Fe–Ni alloys with increasing concentration c Fe is to a large extent associated with the deviation of the dependence of a( c Fe) from Vegard's law.

On the problem of two magnetic correlation lengths in an invar FeNi alloy above TC

The magnetic phase transition in the iron-nickel FCC alloy Fe 70Ni 30 is investigated using a three dimensional analysis of the neutron beam polarization. Thermal hysteresis of the magnetic system parameters was found in the critical regime above T C. A strong effect of the magnetic field on the value of depolarization as well as the width of the hysteresis loop is demonstrated.

Theoretical study of the spin dynamics in Fe-Ni Invar alloys

A Green's function technique is used to study the spin dynamics in the Invar alloys. The longitudinal dynamical structure factor has been calculated. It exhibits a three-peak structure: two peaks due to magnetovolume interactions and a central peak due to the coupling of the transverse mode and the longitudinal relaxing mode. The dependence of the three peaks on temperature T, wave vector k, magnetic field H, and Ni concentration is discussed. The results are in good agreement with the experimental data for the Fe-Ni Invar alloys.

Structure and magnetic properties of Fe1−xNix/Cu Invar superlattices

We have prepared by sputtering techniques a series of fcc [Fe1−xNix/Cu]×10 superlattices with sublayer thicknesses of 3 nm and with Ni concentrations x ranging from 0.26 to 0.54. The use of MgO single-crystal substrates and Cu sublayers in the superlattice growth ensures a well-defined fcc crystal structure in the Fe–Ni sublayers with a Ni concentration as low as 26 at. % and down to liquid helium temperatures. The magnetization of the Fe–Ni sublayers in the superlattices starts to deviate from the well-known Slater–Pauling curve at 40 at. % Ni, and continues to drop until the fcc–bcc transition is completed. A strong dependence of the magnetization on temperature was also observed for the Fe–Ni sublayers in the Invar range, consistent with the behavior of bulk Fe–Ni Invar alloys.